1. Field of the Invention
This invention relates to the field of machining metal surfaces as is done with lathes or similar machines.
2. Descrition of the Prior Art
A well known method of shaping bars of metal, e.g., steel, aluminum, copper alloys, and ceramics and plastics is by lathe turning. The lathe consists of a rotating vice or chuck which grips the bar so that the bar rotates about its axis. A tool bit is mounted on a movable tool bit holder so that as the bar rotates, the toolbit bit moves tangential to and in contact with the surface of the bar. In this mode of use, the tool bit will travel parallel to the axis of the bar which is also the axis of rotation.
If the metal to be machined is a flat metal plate and it is desired to reduce the thickness of the plate, the chuck is replaced by a "face plate". The metal to be shaped is attached to the faceplate by bolts or vacuum means. The faceplate rotates about an axis perpendicular to its surface and the toolbit moves tangential to the surface of metal so as to reduce its thickness.
The process described above is used commonly as an early step in manufacturing a thin metal disk for use in magnetic recording. The purpose of this step is to remove a thin skin (about 0.005 inches thick) which removes surface imperfections and scale; to generate a surface that is flat and to provide a surface finish of 1 to 15 microinch arithmetic average roughness.
An important factor which determines the surface finish is the shape of the tool bit, i.e., the angle that is formed by the two sides of the cutting edge, and the orientation of the cutting edge in relation to the direction of motion of the machined surface. Another factor is the hardness of the tool bit and for this reason, materials are selected for tool bits which provide the greatest resistance to wear and breakage. As the cutting edge becomes dull, it is necessary to interrupt the machining operation to adjust the cutting edge to a new (unused) position and to eventually remove the tool bit and resharpen the cutting edge. The tool bit removes metal by a shearing action which results in a chip forming and sliding on the surface of the tool bit away from the point of cutting. Heat is generated by two factors. One is the work required to separate the chip from the base metal. The second source of heat is the rubbing of the chip against the surface of the tool bit, as it is forced away from the point of formation. The result is that the temperature of the tool bit rises. The hottest point on the tool bit is on the surface a short distance from the cutting edge--not at the edge per se. Pitting begins at this point and eventually leads to breakage, i.e., dulling of the tool bit.
Therefore, another factor that determines the finish of the machined surface is the lubricant that is applied during the cutting operation. The lubricant cools the tool bit and metal so as to relieve the thermal stress and promote nonadherent sliding of the chip. Not only is the composition of the lubrication important, but the method of application is also important. In fact the major problem with most lubrication systems is that the lubricant is not applied where it will do the most good.
In conventional lubrication, the tool bit is positioned between the chip and newly formed surface and the lubricant is applied on the side of the chip away from the tool bit and on the area of metal surface that has not yet been machined. It is not applied directly into the interface between tool bit and chip.
It is an objective of this invention to apply lubricant directly into the interface between toolbit and chip at the cutting edge.
As an early step in the preparation of substrates for magnetic recording according to present practice, the disk is machined with a carbide tool bit (tungsten carbide) shaped like the section of a cone (i.e., a conical section bounded by two flat surfaces). The disk is cut by the sharp edge formed by the flat surface and the conical surface. As the cutting edge becomes dull, the cone is rotated so as to present a sharp unused length of the cutting edge. The entire cutting edge may be resharpened by removing the tool bit and lapping the flat surface. If breakage or wear at the edge is excessive, the tool bit must be discarded.
In the manufacture of disks for magnetic recording, two competitive processes are used to provide the final finish. In one process, the disks are polished with polishing pads of the appropriate composition. In the second process, the surface is mahined using a diamond tool bit with a number of cutting edges (eight is typical). As the tool wears, the operator turns the tool bit so as to present an unused edge. The diamond machining technique removes about 0.002 inches from the disk surface.
There are two actions associated with the cutting mechanism of the diamond tool bit with regard to its ability to provide a fine finish. One is the shearing action performed by the leading length of edge as the tool bit plunges into the metal. The second is the burnishing action of the trailing edge as it passes over the newly formed metal surface. These two mechanisms will be considered subsequently in connection with the present invention.
Proponents of machining with diamond point out that diamond retains its sharp cutting edge longer than, e.g., carbide, so that precision cuts can be made leaving the surface free of stresses that are otherwise generated by a dull tool. Proponents of pad polishing argue that machining with diamond is slow and that diamond tool bits are expensive.
It is a further objective of this invention to provide a tool bit configuration that is amenable to the making of tool bits from a number of different materials so as to provide a tool bit that is almost as hard as diamond but the material is not so expensive.
It is a further objective of this invention to provide a tool bit configuration wherein the conduction of heat away from the cutting edge is improved by judicious selection of the tool bit material.
It is a still further objective of this invention to configure the tool bit and mount it in position on the tool bit holder so that the tool bit cutting edge may be changed very quickly by small increments and, if so desired, automatically between cuts so as to reduce markedly the time of interruption of production.
The machining of titanium presents a special problem because the tool bit wears very quickly. It is a further objective of this invention to provide a tool bit configuration wherein the cutting edge can be changed continually during the cutting process so that the cutting edge is sharp during an entire cut that is long and on materials that cause excessive wear of the tool bit.
It is yet another objective of this invention to provide a configuration of tool bit that is very amenable to resharpening and that can be resharpened many more times than is customary with conventional tool bits.
There are a number of reasons why it is desireable to reduce the amount of lubricant used in a machining operation.
Many lubricants are expensive--especially those that are not waterbased.
The disposal of lubricants is an expense especially when it is done in such a way so as to protect the environment.
Fumes of lubricants are unhealthy, especially for an operator standing over a lathe for many hours. The manner in which lubricants are applied according to present practice often causes a very smoky dense environment. Another objective of this invention is to reduce the amount of lubricant required in the machining operation which would alleviate these problems.
The tool bit configuration and lubrication technique can be used to machine various shapes such as cylinders, plates, etc., and present advantages obvious to those skilled in the art after studying the following descriptions and drawings.
According to present practice, disks for magnetic recording are first machined on a lathe, as described, and then transferred manually to a polishing machine that consists of a conveyor that transfers the disks from one polishing station to the next, in about six steps, so as to provide a finish that is about 0.5 microinch, arithmetic average. Because of the complexity required for an apparatus that would transfer a disk to a lathe, then from the lathe to a polishing machine, no such machine is reported to have been built or put into practice.
There have been built and in use, devices called "stackers" on wheels, that permit loading of discs, one at a time in one end so that the stack of discs can be rolled to a desired position and unloaded from the same end. The "stacker" is comprised of threaded rods, so that each disk is supported on one thread of each of the threaded rods. "Stackers" can be loaded and unloaded from the top only.
It is an object of this invention to provide an apparatus which incorporates the novel toolbit, machining and transfer configurations so as to succeed in automatically manufacturing discs with a very fine finish at a cost that is much lower than is presently achieved.